Q: How do you revalidate data after a mutation in Next.js?

After a Server Action mutates data, call revalidatePath() or revalidateTag() to invalidate the affected caches, or router.refresh() on the client, so the UI reflects the change. Redirecting with redirect() also re-renders the target route fresh. <text

Question 1

How does client-side navigation work in Next.js (next/link and useRouter)?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Easy

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs shipped with the package and, in the **Verified** section, against real output from a build, a running server or a real browser.

## 1. Why This Even Matters — A Story First

Question 2

What is not-found.js in Next.js and how do you trigger a 404?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Easy

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs shipped with the package and, in the **Verified** section, against real output from a build, a running server or a real browser.

## 1. Why This Even Matters — A Story First

Question 3

What happens when you run next dev, next build and next start?

Accepted Answer

**Target Audience:** Beginners preparing for Next.js interviews — no prior Next.js knowledge assumed.
**Difficulty:** Easy

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs shipped with the package and, in the **Verified** section, against a real `create-next-app@16.4.2` project run under all three commands and probed with `curl`.

## 1. Why This Even Matters — A St

Question 4

How do you handle forms with Server Actions in Next.js (progressive enhancement)?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified bloc

Question 5

What is generateStaticParams in Next.js and how does it enable SSG for dynamic routes?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified bloc

Question 6

What is next/script and what are its strategy options in Next.js?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified bloc

Question 7

What changed for next/image defaults in Next.js 16?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes you have used `next/image`.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs (the `image` reference and the version 16 guide) and the built-in defaults in `next/dist/shared/lib/image-config.js`, and, in the **Verified** section, against `curl` requests to the i

Question 8

What breaks when you upgrade to Next.js 16, and how do you migrate?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — useful for anyone who has to plan a 15 to 16 upgrade.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 upgrade guide shipped in `node_modules/next/dist/docs/` and, in the **Verified** section, against real `next build` and `next start` output from a 16.4.2 lab app written i

Question 9

App Router 15 — why are params and searchParams now Promises?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified bloc

Question 10

How do you revalidate data after a mutation in Next.js?

Accepted Answer

After a Server Action mutates data, call revalidatePath() or revalidateTag() to invalidate the affected caches, or router.refresh() on the client, so the UI reflects the change. Redirecting with redirect() also re-renders the target route fresh.

NEXT.JSDECODED.

Easy

How does client-side navigation work in Next.js (next/link and useRouter)?

What is not-found.js in Next.js and how do you trigger a 404?

What happens when you run next dev, next build and next start?

Medium

How do you handle forms with Server Actions in Next.js (progressive enhancement)?

What is generateStaticParams in Next.js and how does it enable SSG for dynamic routes?

What is next/script and what are its strategy options in Next.js?

What changed for next/image defaults in Next.js 16?

What breaks when you upgrade to Next.js 16, and how do you migrate?

App Router 15 — why are params and searchParams now Promises?

How do you revalidate data after a mutation in Next.js?

How do you handle redirects and rewrites in Next.js?

Hard

What changed about caching defaults in Next.js 15?

What is the Full Route Cache in Next.js?

How do you self-host Next.js across multiple instances (cache handlers, ISR, build IDs)?